1. Field of the Invention
This invention relates generally to Magnetic Resonance Angiography signal processing and more specifically to a method of reducing three-dimensional Magnetic Resonance Angiography or stationary tissue data to a two-dimensional image using statistical manipulation, while reducing noise and/or increasing contrast.
2. Description of Related Art
Known techniques for magnetic resonance angiography include methods that, either by volume or sequential-slice acquisition, produce data sets with spatial resolution in three dimensions. These techniques may utilize either time-of-flight effects or motion-induced phase shifts to produce flow contrast. Since there is at this time no practical real-time three-dimensional display technology (e.g., holographic video) available, this data is reduced to two spatial dimensions for convenient viewing by the radiologist. This has generally been accomplished by generating a series of projection images at sequential view angles. These images are then repeatedly displayed in rapid sequence on a display screen to give the viewer a three-dimensional impression of the vascular anatomy. The projection images are created by ray tracing through the volume of data, using some predetermined method for calculating the projected pixel value from each ray. In the case of phase contrast data, it is desirable to preserve the quantitative nature of the data insofar as is possible.
The most obvious projection method is to take the mean of all volume pixel ("voxel") values along each cast ray. This unfortunately yields images with low vessel contrast and poor detection of fine features and vessels, especially for time-of-flight data that always contain signals from unsuppressed stationary tissue. This is because vessels are sparse within the data volume and the total noise and/or background signal along a ray can easily be comparable to or much larger than the total flow signal along that ray. To provide good vessel contrast, the method of choice has become projection of the largest voxel value along each ray. (See Dumoulin, C. L., Souza, S. P., Walker, M. F., and Wagle, W., "Three-dimensional phase contrast angiography," Magn. Reson. Med., (1989), and Armatur, S. C., Masaryk, T. J., Modic, M. T.: et al., "3DFT time-of-flight magnetic resonance angiography," Dynamic Cardiovascular Imaging, 2. 170, 1989). This method is generally known as Maximum Intensity Projection (MIP). The MIP method uses the maximum voxel value along a projection ray and discards the remaining voxel values along that ray. Since along each ray which actually intercepts one or more vessels the only voxel which contributes to the projected pixel is within a vessel, the contrast is very good. Unfortunately MIP has several drawbacks. Since the MIP method retains only one voxel along each ray, it does not make efficient use of the acquired data. This is especially true where a vessel extends over many voxels and averaging over the vessel might be employed to improve the vessel signal-to-noise ratio. The MIP method tends to lose contrast for vessels with strong signal near the walls but weak signal near the axis of the vessel. This situation occurs in phase contrast MR angiography when the velocity at the center of the vessel exceeds the maximum for which the applied flow encoding produces monotonically increasing signal. The MIP method also may obscure real pathological features. For example, this will occur when a stenotic lesion distorts a vessel wall and the lesion appears along the line-of sight rather than displayed in profile. There is a need for a projection method that retains the maximum amount of information possible when transforming the acquired data to projection data while introducing no artifactual features and discarding few actual features.